1. Field of the Invention
The present invention relates to a light scanning device that emits a light beam to a scanned surface of a latent image holding body so as to form a latent image on the scanned surface. The present invention also relates to an image forming apparatus that has this light scanning device and forms an image by an electrostatic printing process.
2. Description of the Related Art
An image forming apparatus has a light scanning device that emits a light beam to a scanned surface of a latent image holding body so that a latent image can be formed on the scanned surface of the latent image holding body. Examples of this type of image forming apparatus are a copier, a printer, a facsimile machine, a plotter, and the like. The image forming apparatus can be roughly classified as follows.
As a first classification, cited is a mono-color (e.g., monochrome) image forming apparatus. In this image forming apparatus, one light source emits a light beam to one latent image holding body (e.g., a photosensitive body drum) to form a latent image thereon. The latent image formed on the latent image holding body is developed by a developing agent (e.g., black toner) to turn the latent image into a visible image. Thereafter, the developed image is transferred to a transferred material (e.g., paper) by transferring means, and the transferred image is fixed by fixing means.
As a second classification, cited is a color image forming apparatus that produces a multi-color image. In this image forming apparatus, one light source emits a light beam to one latent image holding body (e.g., a photosensitive body drum) to form a latent image thereon. The latent images formed on the latent image holding body are developed by a plurality of developing agents (e.g., yellow toner, magenta toner, cyan toner, black toner) to turn the latent images into visible images of respective colors. The visible images of the respective colors are transferred to a middle transfer body by first transferring means such that the visible images of the respective colors are superposed on each other. The visible image transferred on the middle transfer body is then transferred to a transferred medium (e.g., paper) by second transferring means. Thereafter, the image transferred on the transferred medium is fixed by fixing means so as to produce the multi-color image.
As a third classification, cited is another type color image forming apparatus that produces a multi-color image. In this color image forming apparatus, a plurality of light sources emit light beams, respectively, to latent image holding bodies (e.g., photosensitive body drums) that are arranged side by side. In this light beam emission, latent images are formed on the latent image holding bodies, respectively. The latent images formed on the respective latent image holding bodies are developed by respective developing agents (e.g., yellow toner, magenta toner, cyan toner, and black toner) so as to turn the latent images into visible images. Thereafter, a transferred material (e.g., paper) held by a transfer carrying belt or the like are successively sent to a transfer unit of each latent image holding body. In this manner, the visible images of respective colors formed on the respective latent image holding bodies are transferred to a transferred material (e.g., paper) such that the visible images of the respective colors are superposed on each other. The image transferred on the transferred material is then fixed to produce the multi-color image.
As a fourth classification, cited is another type color image forming apparatus. In this color image forming apparatus, a plurality of light sources emit light beams, respectively, to a plurality of latent image holding bodies (e.g., photosensitive body drums) that are arranged side by side. In this manner, latent images are formed on the latent image holding bodies, respectively. The latent images formed on the respective latent image holding bodies are developed by respective developing agents (e.g., yellow toner, magenta toner, cyan toner, and black toner) to turn the latent images into visible images. Thereafter, the respective visible images are transferred to a middle transfer body by first transferring means such that the visible images of the respective colors are superposed on each other. Then, a transferred material (e.g., paper) held by a transfer carrying belt or the like is sent to a second transfer unit of second transferring means. In this manner, the image formed on the middle transfer body is transferred to the transferred material by the second transferring means. Then, the image transferred to the transferred material is fixed by fixing means.
As for the light scanning device that is mounted on the image forming apparatus having any one of the above-described structures, it is demanded that this light scanning device should have fine position characteristics as described in the following. FIGS. 1A through 1F show positions of an actual image that are displaced from an ideal position.
FIG. 1A shows position displacement in a direction corresponding to a sub-scanning direction (hereinafter, referred to as the sub-scanning corresponding direction). In the state shown in FIG. 1A, a scanning line is displaced in parallel from an ideal scanning line. This displacement of the scanning line is caused by sub-scanning corresponding direction characteristics of optical elements, geometric arrangement accuracy of the optical elements, and thermal expansion.
FIG. 1B shows a scanning line inclined in the sub-scanning corresponding direction. In the state of FIG. 1B, the scanning line is inclined from an ideal scanning line in the sub-scanning corresponding direction. This inclination of the scanning line is caused by sub-scanning corresponding direction characteristics of the optical elements, and the geometrical arrangement accuracy of the optical elements.
FIG. 1C shows a scanning line that is curved in the sub-scanning corresponding direction. In the state of FIG. 1C, the scanning line is curved from an ideal scanning line in the sub-scanning corresponding direction. This curve of the scanning line is caused by the sub-scanning corresponding direction characteristics of the optical elements, geometrical shape accuracy of the optical elements, and deformation of the optical elements.
FIG. 1D shows a scanning line that is displaced in a direction corresponding to a main scanning direction (hereinafter, referred to as the main scanning corresponding direction). In the state of FIG. 1D, an image writing start position is displaced each time scanning is performed (image forming position displacement). For the easy illustration of FIG. 1D, the scanning line of which start position is displaced is offset in the sub-scanning corresponding direction. One cause of this image writing start position displacement is that N surfaces of a polygon mirror of the light scanning device have different surface inclinations. Another cause is that light amounts of respective image forming modes are different. Still another cause is that laser wavelengths of N laser diodes are subtly different in multi-beam scanning (that is a method in which N scanning lines are formed in the sub-scanning corresponding direction by using the N laser diodes by one scanning operation).
FIG. 1E shows magnification change in the main scanning corresponding direction. In the state of FIG. 1E, the length of the scanning line in the main scanning corresponding direction is different from the length of an ideal scanning line. This magnification change is caused by the sub-scanning corresponding direction characteristics of the optical elements, the geometrical arrangement accuracy of the optical elements, and thermal expansion. Furthermore, another cause is that wavelengths of N laser diodes are subtly different in multi-beam scanning (that is a method in which N scanning line are formed in the sub-scanning direction by using the N laser diodes by one scanning operation).
FIG. 1F shows the scanning speed difference in the main scanning direction. In the state of FIG. 1F, the scanning speed is microscopically different in the main scanning direction, so that the light beam is not written in an ideal position in the main scanning direction. This scanning speed difference is caused by the main scanning direction characteristics of the optical elements, the geometric arrangement accuracy, and thermal expansion.
As for the state of FIG. 1A, the light emitting timing is adjusted with respect to the sub-scanning corresponding direction so as to be adjusted with respect to the end part of the paper coming to the transfer unit. Accordingly, at the side of the main body of the image forming apparatus, it is enough to perform adjustment to the degree that the light scanning device does not interfere with members or elements of the main body of the image forming apparatus. Thus, high accuracy adjustment is not needed at the side of the main body of the image forming apparatus. In the color image forming apparatuses as in the above-mentioned third and fourth classifications, detection means (using a light beam or an image) for setting the light emission timing need to be provided for each color at the main body of the color image forming apparatus.
As for the state of FIG. 1B, out of the mono-color image forming apparatus as in the above-mentioned first classification, the mono-color image forming apparatus that does not require high position accuracy can obtain necessary scanning inclination characteristics by attaining the necessary accuracy or the like of members and/or elements of the light scanning device. On the other hand, in the case of the image forming apparatus that requires relatively high position accuracy, a parallel degree is adjusted at an attachment part where the light scanning device is attached to the main body of the image forming apparatus, in order to obtain the necessary inclination characteristics of an ultimately formed image. Furthermore, in the case of the light scanning device mounted on the image forming apparatuses as in the above-mentioned third and fourth classifications, a scanning inclination adjusting mechanism moves (or rotates) a reflection mirror around the axis orthogonal to the main scanning corresponding direction and orthogonal to the reflection surface in order to perform inclination adjustment in the light scanning device and to adjust a parallel degree of the light scanning device with respect to the main body of the image forming apparatus.
As for the state of FIG. 1C, out of the image forming apparatuses of the above-mentioned first and second classifications that have a single optical path, the image forming apparatus that does not require high position accuracy can realize the necessary scanning line curve characteristics by attaining the necessary accuracy or the like of members and/or elements of the light scanning device. In some of the image forming apparatuses of the above-mentioned third and fourth classifications that require relatively high position accuracy and include the light scanning device having a plurality of optical paths, a center part of the optical element that has a function of adjusting a scanning line position with respect to the sub-scanning corresponding direction is deformed so as to adjust the scanning line curve.
As for the state of FIG. 1D, light detecting means that include a photo diode element or the like are provided in the light scanning device or at the main body of the image forming apparatus so as to be positioned at an area outside an image forming area. By using as a reference timing a timing at which light passes the light detecting means, the light scanning for forming an image based on image information starts. That is, by using this reference timing, the writing start position in the main scanning corresponding direction is determined. In the case where the writing start position displacement is caused by respective different inclinations of the N surfaces of the polygon mirror, the accuracy of the members and/or elements are improved to the level at which the writing start position displacement does not occur. In the case where the writing start position displacement is caused by the light amount difference in the image forming modes, or caused by the laser diode wavelength subtle difference in the multi-beam scanning, the writing is started at the timing that is adjusted in accordance with light amounts of the respective image forming modes.
As for the state of FIG. 1E, out of the mono-color image forming apparatus as in the above-mentioned first classification, the image forming apparatus that does not require high position accuracy, the necessary expansion characteristics can be obtained with accuracy including the thermal expansion of the members and/or elements of the light scanning device. On the other hand, in some of the image forming apparatuses that require relatively high position accuracy, the light detecting means that include a photo diode element is provided in the light scanning device or at the main body of the image forming apparatus so as to be positioned at both a writing start side and a writing end side outside the image forming area. The magnification is calculated based on the ratio between a reference time period and a detected time period that is a period from the time the light passes the light detecting means at the writing start side to the time the light passes the light detecting means at the writing end side. In this manner, the magnification in the main scanning corresponding direction is adjusted such that the image frequency is changed so as to agree with the reference time period. Further, in the case of the color image forming apparatuses as in the above-mentioned third and fourth classifications, the magnification difference between the optical paths directly leads to image quality degradation such as color difference. Accordingly, in such a color image forming apparatus, in the case where the optical elements are made of a material such as resin having a refractive index that is greatly changed by temperature, it is essential to provide the above-described two light detecting means provided at the writing start side and the writing end side.
As for the state of FIG. 1F, out of the mono-color image forming apparatus, the image forming apparatus that does not require high position accuracy can obtain necessary scanning speed uniformity with accuracy including the thermal expansion of the members and/or elements of the light scanning device. Meanwhile, in the case of the color image forming apparatus (as in the above-mentioned third classification) that does not require high absolute position accuracy, the same optical path is used for the respective colors, so that the difference in the scanning speed uniformity between the colors does not occur. Accordingly, in the case of such a color image forming apparatus of the second classification, it is possible to obtain the necessary scanning speed uniformity with the accuracy including the thermal expansion of the members and/or elements of the light scanning device. However, in the case of the high accuracy plotters in the above-mentioned first and second classifications that require absolute position accuracy, and in the case of the color image forming apparatuses in the above-mentioned third and fourth classifications that use the different optical paths for the respective colors, the changed image frequency is provided in the scanning region, but it is difficult to realize microscopically successive variation of the image frequency. As a result, there is a possibility that a problem such as an image including an undesirable line caused by a lack of dot pitch uniformity in the main scanning corresponding direction occurs. Furthermore, in the case where the optical elements that have a function of adjusting the position of the scanning line in the main scanning corresponding direction are made of a material such as the resin having a refractive index that is greatly changed by temperature, there is a possibility that the temperature distribution in the optical element causes the scanning speed uniformity to be greatly changed. Accordingly, it is necessary to make many different image frequency patterns in the scanning region.
As described above, adjusting means for the position displacement shown in FIGS. 1A through 1E have almost been established. On the other hand, the adjustment for the position displacement shown in FIG. 1F in the case of the resin made optical elements needs to be performed with high accuracy, but a method of adjusting the scanning speed uniformity with high accuracy has not been established yet.
Furthermore, when the light scanning device is mounted on the image forming apparatus, the above-described characteristics are changed due to flatness characteristics of an attachment part of the main body of the image forming apparatus, and the subtle position relation between the light scanning device and the latent image holding body or the transfer unit. Further, in the case where the optical elements having a function of adjusting the position of the scanning line in the main scanning corresponding direction are made of a material such as the resin having a refractive index that is greatly changed by temperature, the above-described characteristics are changed as the temperature of the light scanning device changes. As a result, the image quality is degraded.